The increasing dynamic requirements, such as monotonic start-up, recovery after short-circuit, load transient performance, has let to the re-partitioning of the control structure for many modern insulated power converters. Previously the normal way of structuring the design was to place the control circuit on the primary side and only transmit an error signal from a voltage control system on the secondary side. The most efficient way to meet the requirements mentioned above is to place the control-circuit on the secondary side, where the output voltage can be monitored more efficiently. The introduction of digital control and a digital interface placed on the secondary side will make it even more logical to use this secondary side control.
One problem with this secondary side control is that the control circuit, now placed on the secondary side, has to be biased from the primary side. To achieve this, the input voltage has to be monitored both accurately, since monitoring capabilities of the input voltage is often a requirement in digital controllers and the signal is used in the control of the main converter. The input voltage must also be monitored with a high dynamic bandwidth, for the converter to be able to handle input voltage transients (voltage mode feed-forward).
The control circuit is often biased from a small auxiliary converter, also called a bias regulator, often a flyback type, which bias both the control circuit and synchronous mosfets and the primary switching mosfets.
One method to generate a signal corresponding to the input voltage is to derive it from the bias supply by peak rectifying the forward pulse in the auxiliary converter, or the actual power train, and storing this information in a capacitor. If the primary voltage increases, the capacitor is charged on a cycle to cycle basis and will accurately mirror the primary voltage. If the primary voltage decreases there is no discharge path that discharges the capacitor to the transformed voltage. The capacitor is therefore discharged via a resistor or a current source and the correct voltage is achieved by the charging of the capacitor every cycle. This causes a trade-off between high dynamic performance, which causes a high ripple due to a large discharge off the capacitor every cycle, and accuracy, which requires the capacitor to be discharged only slightly for every cycle.
The drawback of the solution using peak rectification above is that it is hard to match the requirement for high dynamic performance with high accuracy. The solution will always be able to have a high dynamic bandwidth when the input voltage increases but the bandwidth will be limited by the maximum acceptable voltage ripple on the secondary voltage, since the depletion of the capacitor between switching cycles will represent itself as a ripple on the mirrored voltage. The forward voltage drop of the rectifying device will also affect the accuracy of the measured voltage.
Another more costly method is to use linear optocouplers. The input voltage signal is measured by an operational amplifier and converted to a current, which is used to drive a linear optocoupler. This current is then converted to a voltage on the secondary side. One drawback with the optocoupler approach is that one extra component crossing the insulation barrier is required. Also, such a solution is relatively expensive and will also have problem with ageing.